1. Field of the Invention
The present invention relates to a method for preparing a particulate carrier for an olefin polymerization procatalyst which comprises a transition metal compound reacted with the carrier, in which method:
(a) providing a heated melt of a complex compound having formula I EQU MgCl.sub.2.nROH.mED (I) PA1 (b) feeding the heated melt to an atomizing nozzle, PA1 (c) spraying the melt to be atomized from the nozzle into a chamber at a lower temperature than the melt, thereby distributing it in the form of atomized melt, and then solidifying to fine carrier particles and PA1 (d) recovering the fine carrier particles. PA1 (a) a solution essentially containing magnesium dichloride dissolved in ethanol, in an amount of 100 to 300 g of magnesium dichloride/1 of solution, whereby the water-content of the solution does not exceed 5% by weight; PA1 (b) spray-drying of this solution by spraying it into an essentially non-aqueous flow of nitrogen gas, the purity of which is at least 99.9% and the inlet temperature of which flow is between 180.degree. and 280.degree. C., whereby the nitrogen and the solution flows are simultaneously controlled so that the outlet temperature of the gaseous mixture is between 130.degree. and 210.degree. C., provided that the outlet temperature is at least 40.degree. C. lower than the inlet temperature and that the ethanol does not evaporate completely, whereby spheroidal magnesium dichloride particles are obtained; PA1 (c) reacting the magnesium dichloride particles with a titanium halide, which is in vapor or liquid form and optionally diluted with an inert solvent to be evaporated; PA1 (d) recovering the reaction product particles by physical means when they contain 0.7 to 12% by weight of titanium bound to the solid material; and PA1 (e) mixing these reaction product particles with an organometallic compound, which is an alkylaluminum or an alkylaluminum halide. PA1 (a) providing a heated melt of a complex compound having formula I EQU MgCl.sub.2.NROH.mED (I) PA1 (b) feeding the heated melt into an atomizing nozzle having at least one orifice; PA1 (c) spraying the heated melt from the atomizing nozzle into a chamber colder than the melt, distributing the heated melt in the form of an atomized melt, and cooling and solidifying the atomized malt to fine carrier particles; and PA1 (d) recovering the fine carrier particles; wherein an electrical charge is applied to the heated or heated and atomized melt by an electrode arranged in connection with the atomizing nozzle, the voltage of which electrode is about 0.5 to 10 kV.
in which ROH is a lower (C.sub.1-6) aliphatic alcohol, ED is an electron donor, n is 1 to 6, and m is 0 to 1,
2. Description of the Related Art
For the polymerization of olefins, the Ziegler-Natta catalyst system is commonly used, which consists of a so-called procatalyst and a cocatalyst. The procatalyst is based on a compound of a transition metal belonging to any of the groups IVA-VIII (Hubbard) of the Periodic Table of the elements. The cocatalyst is based on an organometallic compound of a metal belonging to any of the groups IA-IIIA (Hubbard) of the Periodic Table of the elements.
Currently, the procatalysts typically comprise an inert carrier, on which the actual active catalyst component, that is, the transition metal compound or the mixture or complex formed by catalytic compounds, is layered. The morphology, size and size distribution of the particles of the carrier influence significantly the properties of the polymer achieved by the catalyst. In fact, by an active catalyst a polymer can be produced, from which, thanks to its purity, no catalyst residues need to be removed.
The morphology and particle size of the carrier, on the other hand, affects the morphology of the polymer product itself, since it has been observed that the morphology of the catalyst is repeated in the structure of the polymer (the so-called replica phenomenon). If a flowable polymer product is desired, having a useful morphology and a narrow particle size distribution, which is desirable in view of the use of the polymer in subsequent many processes, the properties of the carrier shall, because of the replica phenomenon, be made similar to the desired polymer properties.
Currently, procatalysts of the Ziegler-Natta type typically comprise a magnesium-based carrier, such as a magnesium chloride, treated with a transition metal compound, such as a titanium halide, more particularly titanium tetrachloride, and sometimes also with an electron donor compound. It is also known that the carrier can be used in a preferred and equal-sized crystal form by allowing it to crystallize as a complex of any of its crystal solvents.
The patent family consisting of, among others, EP specification 65700 and U.S. Pat. No. 4,421,674 and which claims priority from the Italian application IT 2,188,181 (810521) relates to a method for preparing a catalyst which is particularly active in the polymerization of gaseous ethylene.
In this process, a titanium halide is reacted with a magnesium chloride catalyst carrier, in the form of microspheres, after which the reaction product particles are recovered by physical means and mixed with an organometallic cocatalyst compound.
These conventional methods generally require:
This type of spray-drying method is based on a fairly complete drying of ethanol (C.sub.2 H.sub.5 OH) from the carrier liquid after atomization. The carrier has commonly been dried at a temperature that is above 150.degree. C. whereby a remarkable portion of the alcohol of the complex is evaporated. Typically, the alcohol concentration of the carrier product obtained by the spray drying method is between 12 and 25% by weight, and in any case is below 30% by weight.
One drawback of the conventional spray-drying method is that when the alcohol forms on the surface of the magnesium dihalide, a site reacting with the transition metal compound, the low alcohol content of the carrier results in a weak activation with the transition metal compound, such as titanium halide. A further essential drawback of the spray-drying is the poor morphology and the broad particle size distribution of the carrier obtained, which is caused by the breaking and collapsing of the particles during the process.
According to Finland patent specification 80055 (Neste Oy) the above mentioned carrier complex formed by the carrier and the crystal solvent can be melted to form clear liquid. When this liquid melt is conducted through a spray nozzle into a spraying chamber cooled by cold nitrogen gas, it crystallizes into spheroidal small carrier particles, which are very fluid and loose. In practice, MgCl.sub.2 and C.sub.2 H.sub.5 OH are melted at a temperature of 110.degree. to 130.degree. C. to form a clear melt. This clear, homogenized mixture is fed through an atomizing nozzle into a cooled spraying chamber. The atomizing gas used in the spraying is dry nitrogen, the temperature of which is about +130.degree. C., and the spraying chamber is cooled by conducting in dry nitrogen, the temperature of which is on the order of about -20.degree. C. The nozzle used is a gas-liquid-fluidizing nozzle or the like. Improvements to this so-called spray-crystallization method have been presented, among others, in Finland patent applications 912639 and 915632.
The spray-crystallization method produces very flowable and loose particles. Furthermore, the carrier complex crystallizes without substantial evaporation of the crystallization solvent. The amount of the evaporating crystallization solvent is below 3% by weight of the amount fed into the chamber. When this kind of carrier is brought into contact with the titanium compounds, an abundance of catalytically active complexes between MgCl.sub.2, and the titanium compound are formed on the surface of the carrier when the crystal solvent evaporates. A drawback of this kind of a method is, however, than an insufficient amount of fine particles or particles of equal size are formed. This may be due to the high viscosity and surface tension and/or the agglomeration of the droplets in the atomizing chamber.
RD (Research Disclosure) 218028-A discloses a method in which an electrostatic spraying is carried out in a closed system using at least one high voltage electrode to keep the droplets away from the walls of the atomizing chamber. Grounded needles are placed around the nozzles to discharge the charge of the droplets. A gas flow is also arranged around the nozzle to keep the droplets in the middle zone of the chamber.
The liquid to be sprayed is a solution or suspension that can be spray-dried to solid particles. Typical solutions according to the publication are water or alcohol solutions of magnesium chloride. Typical suspensions are hydrocarbon suspensions of titanium dichloride, on hydrocarbon suspensions of titanium tetrachloride on magnesium chloride particles. Additives can be added to the liquid to improve its conductivity.
This kind of an electrostatic spray-drying technique produces solid titanium dichloride particles, or magnesium chloride particles carrying titanium tetrachloride, having a narrow particle size distribution and being especially suitable for gas phase polymerization.
The above-mentioned electrostatic spray-drying has, however, drawbacks of its own. As is well known, the initial droplets of the spray-drying are small in size and light in weight. In the spray-drying chamber a rapid heating of the droplets and simultaneously a rapid evaporation upwards and outwards takes place, which draws the initial droplets into an uncontrolled turbulent motion, which results in both the agglomeration of the droplets and their attachment to the walls of the chamber.
Accordingly, the electrostatic spray-drying of the Research Disclosure does not relate to any actual improvement of the droplet formation; its main purpose is to prevent the sticking of the formed particles to the process equipment. As stated above, the spray-dried particles intend to break and collapse, so that the electric potential which decreases the surface tension sooner tends to contribute to the breaking and collapsing of the particles, rather than to the forming of morphologically useful particles.
Moreover, large amounts of solvent or suspension liquid evaporates into the chamber space during the spray-drying, which may lead to a discharge of the electrical charge, which can result in a lowering of the atomizing effect and in an electrical breakdown causing an extra risk of explosion.